1. Field of the Invention
The present invention relates to a multilayer bandpass filter having a multilayer structure including a plurality of dielectric layers and a plurality of electrode layers.
2. Description of the Related Art
It is known to manufacture a small-sized low-cost high-frequency bandpass filter by producing a plurality of LC resonators in a multilayer structure including dielectric layers and electrode layers.
Examples of such multilayer bandpass filters are disclosed in Japanese Unexamined Patent Application Publication No. 4-6911 (Patent Document 1), Japanese Unexamined Patent Application Publication No. 2000-201001 (Patent Document 2), Japanese Unexamined Patent Application Publication No. 2003-198226 (Patent Document 3), and International Publication Pamphlet No. 02/009225 (Patent Document 4).
Referring to FIGS. 1A and 1B, a multilayer bandpass filter disclosed in Patent Document 1 is described below.
FIG. 1A shows a circuit diagram, and FIG. 1B shows a cross-sectional view of the multilayer bandpass filter disclosed in Patent Document 1. This filter includes a plurality of LC parallel resonant circuits inductively (magnetically) coupled with each other. More specifically, a plurality of parallel resonators are formed using coils L1, L2, L3, . . . , Ln and capacitors C1, C2, C3, . . . , Cn, such that coils of adjacent resonators are magnetically coupled with each other.
As shown in FIG. 1B, a capacitor electrode pattern 12 and a coil pattern 13 are formed by printing in each of a first layer 10-1, a second layer 10-2 and a third layer 10-3, such that a resonator is formed by these layers. More specifically, capacitance is formed between a ground electrode 11 and capacitor electrodes 12, and coil patterns 13 in two layers are electrically connected to each other via a blind through-hole 14. Below the third layer 10-3, a similar resonator is formed in a plurality of layers including a fourth layer 10-4 and lower layers such that adjacent coils are magnetically coupled.
In a bandpass filter disclosed in Patent Document 2, in a multilayer structure of dielectric layers and electrode layers, a plurality of LC resonators are formed with a plurality of capacitances defined by a plurality of capacitor formation electrodes and inductances occurring on the plurality of capacitor formation electrodes, such that adjacent LC resonators are located at different height levels in the multilayer structure as seen in a thickness direction of the multilayer structure, and such that adjacent LC resonators are electromagnetically coupled with each other. Disposing the plurality of LC resonators at different height levels as seen in the thickness direction in the multilayer structure enables a small-sized bandpass filter to be produced while obtaining a resonator-to-resonator distance necessary to achieve desired characteristics.
In a multilayer bandpass filter disclosed in Patent Document 3, a pair of parallel lines defining first and second filter lines are formed in a partial area of a wiring layer such that the first and second filter lines oppose different circuit layers and extend parallel to the circuit layers, and such that the pair of lines are folded via a dielectric layer.
In a multilayer bandpass filter disclosed in Patent Document 4, two strip lines each defining a resonator are disposed in the same layer such that the two strip lines are spaced by a fixed distance from each other such that the two strip lines are electromagnetically coupled with each other.
In the multilayer bandpass filter disclosed in Patent Document 1, because the coil pattern of each LC parallel resonator is provided in two layers, magnetic coupling between LC parallel resonators becomes large. Furthermore, the two-layer coil pattern causes a reduction in the Q value of the coil, which results in an increase in insertion loss of the multilayer bandpass filter. The above-described problems can be resolved if the distance between LC parallel resonators is set to be sufficiently large. However, this results in an increase in the thickness of the multilayer bandpass filter.
In the multilayer bandpass filter disclosed in Patent Document 2, operations of the resonators are based on self resonance of capacitors, that is, LC resonators are provided by the capacitance of capacitor electrodes and inductance occurring on the capacitor electrodes. However, in this configuration, it is difficult to produce a resonator having desired inductance, and thus, it is difficult to produce a low-loss bandpass filter with desired characteristics.
In the multilayer bandpass filters disclosed in Patent Documents 3 and 4, although it is possible to achieve a small-sized low-loss bandpass filter, in order to increase the number of resonators to achieve steep attenuation at upper and lower edges of a passband, it is necessary to dispose two stages of filters in a thickness direction such that strip lines are coupled with each other in the thickness direction. Thus, a problem in this configuration is that the filter must have a large thickness.
In the known multilayer bandpass filters described above, when LC parallel resonators including capacitor electrodes and inductor electrodes are formed in a multilayer structure such that adjacent inductor electrodes are inductively coupled with each other, a ripple (deviation) of a transfer characteristic occurs in a passband.